Hydrous calcium silicate insulation products manufactured by recycling greenware

ABSTRACT

Off-size greenware (the product resulting from the initial gelation and filter pressing of calcareous and siliceous materials in an aqueous slurry) is recycled. The greenware is pulverized into a desirable size and combined with new calcareous and siliceous materials in the initial slurrying step. The slurry containing recycled greenware gels at an improved rate.

This is a division of application Ser. No. 944,540, filed 9/21/78 nowU.S. Pat. No. 4,477,397.

NATURE OF THE INVENTION

This invention relates to the manufacture of hydrous calcium silicateinsulating products from slurried calcareous and siliceous components.

BACKGROUND OF THE INVENTION

The term hydrous calcium silicate denotes a crystalline compound formedby the reaction of lime (CaO), silica (SiO₂), and water (H₂ O). The twohydrous calcium silicates that generally are of interest are:tobermorite having the formula 4CaO.5SiO₂.5H₂ O; and xonotlite, havingthe formula 5CaO.5SiO₂.XH₂ O. Hydrous calcium silicate products oftenare used as heat insulation materials.

Methods for reacting and drying a molded aqueous slurry of reactivecementitious constituents and reinforcing fibers to form hydrous calciumsilicate insulation products are known in the art. One such methodincludes placing a molded slurry of the reactive cementitiousconstituents and reinforcing fibers in an autoclave, introducingpressurized saturated steam into the autoclave to indurate the slurry,and removing the products from the autoclave. Another such methodincludes placing a molded slurry of the reactive cementitiousconstituents and reinforcing fibers in an autoclave, introducingpressurized saturated steam into the autoclave to indurate the slurry,simultaneously further indurating and drying the slurry with superheatedsteam to convert the slurry to a final product, reducing the pressure inthe autoclave to atmospheric pressure and removing of the product.

In practice the principal slurry constituents, i.e. calcareous andsiliceous materials, reinforcing fibers and water are mixed to form aslurry which is then molded to impart a predetermined shape to theslurry and final product. The slurry is molded or shaped in anyconvenient manner. Generally, however, one of two types of molds isemployed, i.e. pan molds or filter press molds. In pan molds, the slurryremains in the mold while the cementitious materials are reacted toconvert them to a hydrous calcium silicate insulation. A pan moldgenerally defines a mold cavity of a particular shape and dimension;e.g. a flat rectangular pan is used to form flat ware or blocks, whilean arcuate, generally U-shaped mold forms half-section insulation piecesused to form molded pipe covering for insulating pipes, ducts, and thelike. The filter press mold generally comprises a perforated moldingsurface over which the slurry is poured. A perforated mechanical piston,complementary in shape to the mold, compresses the slurry and dewatersit to the point where it is self-supporting. The filter press moldingtechnique is conducted in the absence of any applied heat and underpressure sufficient only to express water from the slurry in the filterpress and to form solid articles such as pipe covering nd flat ware. Atthis point the molded slurry becomes the product defined as "greenware"in this description. The filter press molding technique is described inU.S. Pat. No. 2,699,097.

The greenware is further cured to the desired hydrous calcium silicateproduct by several methods. One method introduces pressurized, saturatedsteam into a closed system or autoclave in sufficient amount to bringthe pressure in the autoclave from 100 to 350 psi at a temperatureranging from 328°-406° F. in 30 minutes or less and preferably within 15to 30 minutes after the introduction of the steam begins. (The term"pressure" as used herein refers to gauge pressure (in pounds per squareinch), i.e. the pressure above that of the atmosphere.).

The greenware is then maintained under this steam pressure in anautoclave for a period of time sufficient to indurate it. After theproduct has been removed from the autoclave, it can be dried in anauxiliary drier, if required. The drying temperature in the auxiliarydrier must be below the decomposition point of the organic fiberreinforcement to achieve the best possible strength of the product.

The other method disclosed herein introduces pressurized, saturatedsteam into a closed system or autoclave in sufficient amount to bringthe pressure in the autoclave from 100 to 350 psi in 30 minutes or lessand preferably within 15 to 30 minutes after the introduction of steambegins. (The term pressure as used herein refers to gauge pressure (inpounds per square inch), i.e. the pressure above that of theatmosphere.) After the introduction of the steam, the temperature in theautoclave is raised by heating coils to a temperature ranging from about407° to about 600° F. to produce the superheated steam.

The greenware is then maintained in the autoclave until a perdeterminedpercentage of the moisture (by weight) of the ware has been removed byevaporation into the superheated steam atmosphere. The free moisture isreduced substantially during the cycle, but never drops below 10% byweight of solids during the entire cycle. While continuing to circulatethe steam in the system, the pressure in the autoclave is reduced toatmospheric conditions within 60 minutes or less and the final productis removed. The product subsequently can be dried in an auxiliary drier,if required. The drying temperatures in the auxiliary drier must bebelow the decomposition point of the organic fiber reinforcement, or abrittle product is obtained. The retention of at least 10% byweight-free moisture in the molded slurry allows for simultaneousindurating and drying in an autoclave at temperatures above thedecomposition point of the reinforcing organic fibers.

An important step in the overall process is the gel formation step priorto molding the slurry. After a mixture of pulverized siliceous andcalcareous materials are suspended in an aqueous slurry sufficient timemust be allotted for the slurry to gel before it can be processedfurther. This period of gelation is necessary to permit the formation ofa thickened slurry. Gelation requires alternate quiescent periods forallowing the formation of nucleated solids followed by gentle agitation.

I have now discovered that the addition of pulverized greenware materialderived from the initial filter press molding of the slurry in theabsence of applied heat substantially decreases the amount of timerequired for this preliminary gelation step.

SUMMARY OF THE INVENTION

Briefly stated, this invention comprises a process for making hydrouscalcium silicate insulation products from aqueous slurries of calcareousand siliceous materials comprising:

(a) preparing a water dispersion of reactive calcareous and siliceouscontaining materials;

(b) gelling the dispersion by heating at a temperature of approximately180°-210° F.;

(c) molding said partially gelled dispersion under pressure andexpressing water therefrom thereby converting the gelled dispersion togreenware;

(d) unmolding said greenware and selecting those greenware articles tobe recycled;

(e) completing the curing of the remaining molded articles;

(f) pulverizing selected greenware articles from step (d) and addingsaid particulated material to the dispersion of step (a) in a subsequentcycle of the process.

DESCRIPTION OF THE INVENTION

The specific lime to silica ratio of the slurry is dependent primarilyupon the desired type of crystalline hydrous silicate desired in thefinal product. For example, if it is desired to obtain a crystallineproduct predominantly composed of a crystalline matrix structure of thetype commonly referred to as xonotlite (5CaO.5SiO₂.XH₂ O), a CaO/SiO₂mol ratio of approximately 1/1 would be utilized in the slurry. If thedesired cyrstalline product is tobermorite having the formula4CaO.5SiO₂.5H₂ O, a CaO/SiO₂ mol ratio ranging from 0.75/1 to 0.80/1would be employed in the slurry. In general, the CaO/SiO₂ molar ratioranges from 0.65/1 to 1.3/1.

Control of the density of the resultant product is primarilyaccomplished by controlling the relative amount of water utilized in themake-up of the slurry. For example, an apparent density of 11 pounds percubic foot, which may be considered a nominal apparent density, would beobtained utilizing a slurry having a ratio of water to total dry solidsof approximately 6:1.

The siliceous materials employed in this invention include portlandcement, siliceous sand, quartz, diatomaceous earth, clays, silica gel,pozzolana, perlite, and the like and mixtures thereof.

The calcareous materials used in this invention include portland cement,quick lime, slaked lime and the like and mixtures thereof.

The organic materials of this invention are cellulosic fibers such asfibers of pulp fiber, cotton, straw, bagasse, wood flour, hemp, rayonand the like. A preferred pulp fiber is bleached softwood pulp.Alkali-resistent glass fibers can be employed with the cellulosicmaterial.

In the method of this invention the cementitious materials noted abovein water are mixed in a hydrapulper to form a slurry and the pulverizedparticulated greenware from a previous cycle is added to the slurrycomposition. The slurry is transferred to a holding tank fitted with anagitator and preferably maintained at a temperature ranging from 60° to90° F. The addition of the particulated greenware has the effect ofshortening the length of time required for the slurry to reach theproper consistency for further processing. The particulated greenware isadded in an amount up to about 30% by weight of the total solidsmaterial. Preferably, it is added in an amount between about 10 and 20percent of the total weight of the solids. A maximum particle size is1/4 inch mesh size. The incorporation of this material into the gelmixture now reduces the gelation time from an average, for example, of105 minutes to 70 minutes. This is, of course, a substantial reductionin the gelation time required.

After the slurry has gelled to a proper consistency it is molded intoarticles by filter press molding techniques. This technique is wellknown to those skilled in the art and as noted previously comprisesmolding the slurry under sufficient pressure to express some of thewater therefrom and to provide solid articles which are the greenware ofthis description. These solid articles are inspected and those articlesfound not to be suitable because of breakage or undesired dimensions areremoved and pulverized to a particle size of not greater than 1/4 inchmesh size.

This molded slurry (greenware) is then subjected to pressurizedsaturated steam in a closed system or autoclave in an amount sufficientto bring the pressure in the autoclave to 100 to 350 psi at atemperature ranging from 328° to 406° F. in 30 minutes or less andpreferably within 15 to 30 minutes after the introduction of the steambegins. The term "pressure" is pounds per square inch, gauge. The slurryis maintained under this pressure for a period of time sufficient toindurate the slurry. Curing to form the final desired hydrated calciumsilicate product is completed by additional heating, for example withsuperheated steam at 407° to 600° F., or other temperatures as will beknown to those skilled in the art.

EXAMPLE

A low density hydrated calcium silicate heat insulation material is madefrom the following materials, in parts by weight of solids.

    ______________________________________                                        MATERIALS       PARTS BY WEIGHT (Dry)                                         ______________________________________                                        Quick lime      34.78                                                         Diatomaceous earth                                                                            21.4                                                          (86 sq. ft. per gram)                                                         Diatomaceous earth                                                                            29.96                                                         (54 sq. ft. per gram)                                                         Wood pulp       6.5                                                           Glass and Rayon fibers                                                                        1.06                                                          Particulated recycled                                                                         6.29                                                          greenware                                                                     ______________________________________                                    

A dispersion of the various materials is made by dispersing the woodpulp in 350 parts by weight of water heated to 200° F. in a hydratedpulper to produce a dispersion. This dispersion of the wood pulp is thenadded to a premixer wherein the hydrated lime is added and mixed for 1minute. Another 900 parts by weight of water at 200° F. is added toanother premixer and the diatomaceous earth and pulverized greenware isadded thereto and mixed for 1 minute. Thereafter the contents of the twopremixers are added to a gel tank wherein the materials are thoroughlymixed for 10 minutes. The resulting slurry or suspension of ingredientsis then premitted to gel and a gelation time of only about 70 minutes isrequired. The gel so produced is then drawn off in small quantities to avolumetric tank in precise quantities for charging a precision typefilter mold shaped to make 3 inch annular pipe insulation of 11/2 inchwall thickness and a length of 36 inches. The ram of the mold compressesthe gel to force the water out through the cylindrical filter formingthe inside surface of the pipe insulation to leave a pipe insulationwhich is self sustaining and handleable. The block is then removed fromthe filter mold and now, in its greenware condition is inspected.Unsatisfactory portions are removed. The remaining block is stacked in arack which when filled is rolled into an autoclave for induration. Afterthe autoclave is sealed, the pressure in the autoclave is raised to 175psi over a 30-minute cycle and the blocks are subjected to saturatedsteam at this pressure for 11/2 hours. Thereafter the temperature in theautoclave is raised by heating coils to 600° F. to produce superheatedsteam which slowly dries the blocks over another 2-hour period. Theautoclave is then depressurized over a 1/2-hour period, and the rackswhich hold the insulation block are removed from the autoclave. Thematerial so produced has a modulus of rupture of 115 psi and a densityof 12.5 lbs. per cubic foot.

While the invention has been described in considerable detail I do notwish to be limited to the particular embodiments shown and described andit is my intention to cover hereby all novel adaptations, modificationsand arrangements thereof which come within the practice of those skilledin the art to which this invention relates.

What is claimed is:
 1. A moldable slurry for manufacturing low densityhydrated calcium silicate heat insulating bodies comprising an aqueousslurry of reactive calcareous and siliceous materials and particulatedgreenware, and produced by a process including the steps of:a. preparinga moldable slurry comprising a water dispersion of reactive calcareousand siliceous containing materials; b. partially gelling said dispersionby heating at a temperature of approximately 180°-210° F.; c. moldingsaid partially gelled dispersion and expressing water therefrom therebyforming rigid greenware articles; d. removing portions of said greenwarearticles selected for recycling to the process; e. curing the remainingmolded articles by heating and pressurizing; and f. pulverizing theselected greenware articles from step d to form particulated greenwareand adding said particulated greenware to the composition of step a in asubsequent cycle of the process.
 2. The slurry of claim 1 wherein saidparticulated greenware is present in an amount of between 10 and 30percent by weight of dry solids.